Turbomachinery abradable seal

ABSTRACT

A centripetal turbine (1) comprises a housing (3) and a turbine wheel (4) mounted within the housing (3) and having turbine blades (5). The housing (3) defines an annular inlet passageway (7) arranged around a portion of the turbine wheel (4), and an outlet passageway (8) which has a generally cylindrical portion arranged around a portion of the turbine wheel (4). The housing (3) also defines a curved annular shoulder (9) curving radially outwards from the generally cylindrical portion of the outlet passageway (8) to the annular inlet passageway (7). The radially outer edge of each turbine blade (5) has a first portion (11) adjacent the generally cylindrical portion of the outlet passageway (8), and a second curved portion (12) adjacent the curved annular shoulder (9). The housing (3) is provided with an annular layer (13) of an abradable material covering substantially all of the substantially cylindrical portion of the outlet passageway (8) but at most only a relatively small annular portion of the curved shoulder (9) adjacent the cylindrical portion of the outlet passageway (8).

TECHNICAL FIELD

The present invention relates to improvements in centripetal turbinesand compressors, and particularly, but not exclusively, turbines andcompressors incorporated in turbo-chargers.

BACKGROUND OF INVENTION

Centripetal turbines generally comprise a turbine wheel mounted within aturbine housing, the inner wall of which defines an annular inletpassageway arranged around the turbine wheel and a generally cylindricalaxial outlet passageway extending from the turbine wheel. Thearrangement is such that pressurised gas admitted to the inletpassageway flows to the outlet passageway via the turbine wheel, therebydriving the turbine wheel.

Where the outlet passageway meets the inlet passageway the inner wall ofthe turbine housing curves radially outwards forming a curved annularshoulder. The radially outer edges of the turbine wheel blades areprofiled to substantially follow the profile of the housing, having afirst portion in the region of the inlet passageway which is typicallystraight, a second curved portion which follows the contour of thecurved annular shoulder, and a third substantially straight portionwhich extends into the outlet passageway.

The turbine blades are designed to follow closely the profile of thehousing in order to minimise the gap between the two which is necessaryto maximise efficiency. However, minimising the gap between the tips ofthe turbine blades and the inner wall of the housing is problematicalbecause of the differential thermal expansion of the various turbinecomponents as the turbine temperature rises to its operatingtemperature.

Conventionally turbines have been constructed with a clearance gapbetween the blade tips and the housing to allow for the differentialexpansion. However, given that turbines are generally designed foroperating over a range of temperatures a compromise must be reached;either a gap large enough to allow for differential expansion at allextreme operating temperatures must be provided, which will result in anundesirably large gap at certain operating temperatures, or only arelatively small clearance gap may be provided and it be accepted thatat least in some, albeit transient, operating conditions the turbineblades will rub against the housing (this could obviously result inrapid wear and in some cases damage to the turbine components).

Various approaches have been adopted to tackle this problem, one suchapproach being to coat the inner wall of the turbine housing with anannular layer of an abradable material adjacent the turbine blade tips,i.e. covering the curved internal shoulder and that part of the outletpassageway which surrounds the turbine wheel. This allows the turbine tobe constructed with essentially zero clearance between the turbine wheeland the housing, with the turbine wheel effectively machining its ownclearance as it rotates. Various different materials have been proposedas suitable abradable coatings, see for example U.S. Pat. No. 5,185,217.

Whilst the above solution is effective, it is also relatively expensiveboth in terms of the abradable materials used and the associatedprocesses of coating the turbine housing with a given abradable layer.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate theabove disadvantages.

According to a first aspect of the present invention there is provided acentripetal turbine comprising a housing, a turbine wheel mounted withinthe housing and having turbine blades, the housing defining an annularinlet passageway arranged around a portion of the turbine wheel, anoutlet passageway which has a generally cylindrical portion arrangedaround a portion of the turbine wheel, and a curved annular shouldercurving radially outwards from said generally cylindrical portion of theoutlet passageway to said annular inlet passageway, the radially outeredge of each blade each having a first portion adjacent the generallycylindrical portion of the outlet passageway and a second curved portionadjacent the curved annular shoulder, wherein the housing is providedwith an annular layer of an abradable material covering substantiallyall of said substantially cylindrical portion of the outlet passagewayand at most only a relatively small annular portion of the curvedshoulder adjacent said cylindrical portion of the outlet passageway.

We have made the surprising discovery that by terminating the abradablecoating at/or adjacent to the annular region where the outlet passagewaymeets the curved shoulder, which represents a significant saving inmanufacturing cost, there is virtually no loss in turbine performance.This is in marked contrast to conventional turbine designs in whichabradable coatings are provided so as to cover the entire surface of theturbine housing adjacent to the turbine blades.

Any suitable abradable material may be used, such as the variousmaterials proposed in the prior art. However, we have found that furthercost savings can be made by using a material which comprises a mixtureof nickel powder with aluminium powder and a binder, in which the nickelcontent is approximately 90% to 96% by weight and the aluminium contentis approximately 3% to 7% by weight. For instance, in a preferredembodiment of the invention the abradable material is a mixturecomprising about 93% nickel by weight, about 5% aluminium by weight, andabout 2% binder by weight. Such a powder is sold by the US company MetcoInc. (of 1101 Prospect Avenue, N.Y. 11590) under the trademark METCO450. This material is significantly cheaper than abradable materialsconventionally used in turbines but has not previously been used inturbines because it has been thought that it would not be abradableenough and indeed might oxidise and harden thereby becoming abrasive.However, we have discovered that this material performs well inturbines, at least at temperatures below about 760° C.

The abradable coating may be applied to the surface of the turbinehousing by any suitable method. In the case of the above preferredabradable material, the abradable layer is preferably applied by theconventional process of thermal spray coating. The application processis controlled so that the abradable layer has an appropriate porositycorresponding to a desired hardness (which may for instance depend onthe material and construction of the turbine blades).

The abradable material may be applied to the surface of the turbinehousing such that a base layer of the coating is relatively hard so thatonly outer regions of the layer are truly abradable. That is, theabradable layer may be applied in such a way that it is effectively onlyabradable up to a certain depth. However, reference to the "abradablelayer" above and hereinafter are to be understood as references to theentire layer of abradable material applied to the turbine housing andnot just that part of the layer which is in practical circumstancesactually abradable. Thus, references to the thickness of the "abradablelayer" below are to be understood as references to the thickness of theentire layer as applied to the turbine housing notwithstanding that thelayer may not be considered to be abradable throughout its entirethickness.

The optimum thickness of the abradable layer will depend to a largeextent on the size of the initial clearance between the turbine wheeland the turbine housing. The abradable coating is preferably as thick aspossible for any given clearance whilst allowing the turbine to beself-starting. Thus the average thickness of the abradable layer ispreferably about 0.1 mm less than the clearance between the turbinewheel and the housing.

For instance, within turbines incorporated in turbo-charges, the radialgap between the extreme tips of the turbine blades and the inner wall ofthe housing is generally less than 1 mm. Thus, for example, in apreferred embodiment of the invention the radial gap between the extremetips of the turbine blades and the inner wall of the housing is about0.5 mm and the thickness of the abradable layer is just less than theclearance gap at, for instance, about 0.4 mm.

In addition to the above detailed first aspect of the present invention,we have also discovered that significant performance improvements can beattained in centripetal compressors by the provision of an abradablecoating on the compressor housing. That is, centripetal compressorsgenerally comprise a compressor wheel mounted in a compressor housingwhich defines a generally cylindrical axial inlet passageway leading tothe compressor wheel and a annular outlet passageway arranged around thecompressor wheel. Although the construction of such compressors isbroadly similar to that of turbines, problems associated withdifferential expansion of the compressor components have not previouslybeen thought significant as the operating temperatures of compressorsare generally substantially lower than the operating temperatures ofturbines. However, we have discovered that measurable improvements inperformance can be obtained by minimising the clearance gap between thecompressor wheel blades and the compressor housing by the provision ofan abradable coating on the surface of the housing adjacent to thecompressor wheel blade tips.

Accordingly, a second aspect of the present invention provides acentripetal compressor comprising a housing, a compressor wheel mountedwithin the housing and having compressor blades, the housing beingprovided with an annular layer of an abradable material in a regionadjacent said turbine blades.

In a preferred embodiment of the compressor the housing defines an inletpassageway which has a generally cylindrical portion arranged around aportion of the compressor wheel, an annular outlet passageway arrangedaround a portion of the compressor wheel, and a curved annular shouldercurving radially outwards from said generally cylindrical portion of theinlet passageway to said annular outlet passageway, the radially outeredge of each blade having a first portion adjacent the generallycylindrical portion of the inlet passageway, and a second curved portionadjacent the curved annular shoulder, and the annular layer of abradablematerial covers at least a part of said curved shoulder adjacent thecompressor wheel blades.

As with the first aspect of the present invention, we have discoveredthat cost savings can be made, without significant detriment toperformance, by applying the abradable coating only to that portion ofthe compressor housing adjacent the compressor wheel blades towards theoutlet of the housing. Thus, in a preferred embodiment of the secondaspect of the present invention the abradable coating covers at least apart of said annular shoulder but all, or substantially all, of saidcylindrical portion of the inlet passageway is not covered by thecoating.

Further savings in cost can be attained by covering only that portion ofthe annular shoulder which lies towards the annular outlet with saidabradable coating. Thus, in a more preferred embodiment of the presentinvention, the abradable coating covers an area of the annular shoulderfor which the curvature has a radial component which is greater than, orsubstantially equal to, its axial component.

The optimum thickness of the coating depends upon the size of theinitial clearance gap between the turbine blades and the housing and ispreferably as thick as possible whilst not preventing the compressorfrom starting under its own power. Typically, the thickness of theabradable coating will lie within the range of 0.1 mm to 0.5 mm.

There are many materials suitable for use as an abradable coating incompressors, which will generally have different specifications frommaterials used as abradable coatings in turbines. We have found that anabradable material that performs well is one comprising a mixture of analuminium alloy powder, silicon and polyester. A preferred compositioncomprises about 60% by weight of the aluminium alloy, about 12% byweight of silicon and about 28% by weight polyester. (Such a material issold by Metco Inc. under the trademark METCO 601).

The above preferred abradable material is preferably applied to thecompressor housing by a plasma jet spray process. As discussed above inrelation to the turbine, the abradable layer may actually be applied tothe housing such that a base portion of the layer is relatively hard andthus not truly abradable. However, references to the thickness of thelayer, both above and hereinafter, are to be understood as references tothe thickness of the layer as applied to the housing regardless ofwhether or not the layer is actually abradable throughout its thickness.

SUMMARY OF THE DRAWINGS

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is an axial cross-section of a turbo-charger incorporating aturbine and a compressor in accordance with the present invention; and

FIG. 2 illustrates a modification of the compressor shown in FIG. 1.

FIG. 3 illustrates a greatly expanded view of an abradable coating usedwith the turbo-charger of FIG. 1, showing the region encompassed bycircle 3--3.

DESCRIPTION OF THE INVENTION

Referring to the drawing, the illustrated turbo-charger is of arelatively conventionally design modified in accordance with the presentinvention. Accordingly, only features relevant to the various aspects ofthe present invention will be described in detail below.

The turbo-charger comprises a centripetal turbine, illustrated generallyby the reference numeral 1, and a centripetal compressor, illustratedgenerally by the reference numeral 2. The turbine 1, comprises a housing3 which houses a turbine wheel 4 which has radially extending blades 5.The housing 3 defines an annular inlet chamber 6 which has an annularpassageway 7 arranged around a rear portion of the turbine wheel 4. Thehousing 3 further defines a generally cylindrical outlet passageway 8 aportion of which surrounds a front portion of the turbine wheel 4. Wherethe outlet passageway 8 meets the inlet passageway 7 the inner wall ofthe housing 3 curves radially outwards defining a curved annularshoulder 9.

The radially outer edge of each turbine blade 5 is profiled such that ithas a rear relatively straight portion 10 which extends across the inletpassageway 7, a front relatively straight portion 11 which extends intothe outlet passageway 8, and a curved portion 12 which follows theprofile of the curved annular shoulder 9.

As discussed in the introduction to this specification, the blades 5 areprofiled so that they closely follow the profile of the housing 3 tominimise the clearance gap therebetween. In the drawing the gap betweenthe turbine blades 5 and the housing 3 is exaggerated to allowillustration of an abradable layer discussed below.

In accordance with the present invention, an annular layer 13 of anabradable material is provided on the surface of that part of the outletchamber which surrounds the turbine wheel, i.e. the internal surface ofthe housing 3 adjacent the portions 11 of each turbine blade 5.

In the preferred embodiment illustrated, the radial gap between theoutermost edges of the turbine blades 5 and the inner wall of thehousing 3 is approximately 0.5 mm and the thickness of the abradablelayer 13 is approximately 0.38 mm.

A variety of abradable materials could be used for the abradable layer13, but in the illustrated preferred embodiment of the invention, theabradable material comprises 93% by weight nickel powder, 5% by weightaluminium powder, and 2% of an organic binder and was obtained from thecompany Metco Inc under the trade name METCO 450/17.

The illustrated turbine differs from conventional turbines provided withan abradable layer, in that all (or substantially all) of the curvedannular shoulder 9 is left uncoated. This leads to a significant savingin the amount of abradable material needed (and thus a significantreduction in manufacturing cost) with very little loss in performance.In fact, in tests performance losses have proved to be too slight toproperly measure.

In addition to the saving on the amount of material used, the presentinvention also provides a saving in cost by utilising a relatively cheapmaterial, i.e. METCO 450/17 powder, which has previously been thoughtunsuitable for use in this application (as discussed above).

The abradable layer 13 may be applied to the surface of the housing 3using any suitable process, for instance by a process of thermal spraycoating. Such a process is well known and thus will not be furtherdiscussed here. The abradable material is applied so that it has aporosity corresponding to the desired hardness, and is preferablyapplied by first forming a relatively hard (and thus relativelynon-abradable) base layer onto which a softer layer is formed. Forinstance, an appropriate hardness for the upper abradable region of thelayer 13 is given by the specification R^(15Y) =70±5.

Referring again to the drawing, the compressor 2 has a similar structureto that of the turbine I and comprises a compressor wheel 14 mounted onthe same axis as the turbine wheel 4 within a housing 15. The housing 15defines a generally cylindrical inlet passageway 16 which leads to thecompressor wheel 14 and a portion of which surrounds a front portion ofthe compressor wheel 14. The housing 15 further defines an annularoutlet chamber 17 which has an annular outlet passageway 18 whichsurrounds a rear portion of the compressor wheel 14. Between the inletpassageway 16 and the outlet passageway 18 is a curved annular shoulder19.

The illustrated compressor 2 differs from conventional compressors inthat an annular layer 20 of an abradable material is applied to thesurface of annular shoulder 19. Provision of the abradable layer 20 hasmade it possible to effectively reduce the clearance between thecompressor wheel 14 and the housing 15 which has produced a measurableimprovement in performance. Tests have shown that providing theabradable layer 20 as illustrated results in about a 4% increase in thepressure coefficient of the compressor 2.

As in the case of the turbine described above, it is not necessary forthe annular layer 20 of abradable material to cover all of the innerwall of the housing 15 adjacent the compressor wheel 14; significantcost savings can be attained (with minimal effect on performance) bycovering only the annular shoulder 19 which leads to the annular outletpassageway 18, as illustrated. Even greater savings can be attained bycovering only that part of the shoulder 19 which lies towards the outlet18. For instance, the abradable layer 20 may cover that region of theannular shoulder 19 which extends from the outlet passageway 18 to aregion at or adjacent the region of the shoulder at which the radialcomponent of its curvature is roughly equal to its axial component. Thisis illustrated in FIG. 2.

It will be appreciated that there are a variety of materials which couldbe used for the abradable layer 20. However, in the preferred embodimentillustrated the abradable material is a powder comprising 60% by weightof aluminium alloy, 12% by weight of silicon, 28% by weight ofpolyester, obtained from the company Metco Inc under the trade nameMETCO 601. This particular powder is chosen because it is soft andabradable enough not to damage the relatively thin blades of thecompressor wheel. This powder has a higher melting point than the METCO450 powder mentioned above, and therefore is applied to the surface ofthe compressor housing by a plasma jet spray process. The plasma jetspray process is a conventional process and will not be discussed indetail here.

The thickness of the abradable layer 20 should be as large as possiblewhilst not preventing the compressor from self-starting. In thepreferred embodiment illustrated the thickness of the layer 20 is about0.5 mm. As discussed above in relation to the abradable layer 13 appliedto the turbine, in practice the abradable material is preferably appliedto the surface of the housing so as to initially form a relatively hard(and thus non-abradable) base layer. That is, the abradable layer willnot be practically abradable throughout its entire thickness.

It will be appreciated that the present invention is applicable toturbines and compressors employed in many different applications and isnot limited to turbo-chargers. Similarly, it will be appreciated thatmany of the details of the turbo-charger illustrated could be modified.

As regards the layers of abradable material, it will be understood thattheir thickness and exact positioning could vary, for example withvarying turbine/compressor structures. For instance, in largerturbo-chargers the clearance between the turbine blades and the housingmay be about 0.8 mm, in which case the thickness of the abradable layeris preferably about 0.7 mm (e.g. about 0.68 mm). In addition, in thecase of the turbine the abradable layer need not necessarily cover allof that portion of the outlet passageway that surrounds the turbinewheel, but could for example terminate before the curved annularshoulder and/or short of the front end of the turbine wheel.

I claim:
 1. A centripetal turbine comprising a housing, a turbine wheelmounted within the housing and having turbine blades each having aradially outer edge, the housing defining an annular inlet passagewayarranged around a portion of the turbine wheel, an outlet passagewaywhich has a generally cylindrical passageway arranged around a portionof the turbine wheel, and a curved annular shoulder curving radiallyoutwards from said generally cylindrical portion of the outletpassageway to said annular inlet passageway, the radially outer edge ofeach blade having a first portion adjacent the generally cylindricalportion of the outlet passageway, and a second curved portion adjacentthe curved annular shoulder, wherein the housing is provided with anannular layer of an abradable material covering substantially all ofsaid substantially cylindrical portion of the outlet passageway and atmost only a relatively small annular portion of the curved shoulderadjacent said cylindrical portion of the outlet passageway.
 2. Acentripetal turbine according to claim 1, wherein the layer of abradablematerial covers only said substantially cylindrical portion of theoutlet passageway.
 3. A centripetal turbine according to claim 2,wherein the abradable material comprises a mixture of nickel powder,aluminum powder and a binder.
 4. A centripetal turbine according toclaim 3, wherein the binder is an organic binder.
 5. A centripetalturbine according to claim 4, wherein the abradable material comprisesfrom about 90% to about 96% by weight of nickel powder and about 3% toabout 7% by weight of aluminum powder.
 6. A centripetal turbineaccording to claim 5, wherein the abradable material comprises about 93%by weight of nickel and about 5% by weight of aluminum.
 7. A centripetalturbine according to claim 6, wherein the abradable material is appliedto the surface of the turbine housing by a process of thermal spraycoating.
 8. A centripetal turbine according to claim 7, wherein theaverage thickness of the abradable layer is about 0.1 mm less than theradial clearance between the turbine wheel and the turbine housing inthe region of the abradable layer.
 9. A centripetal turbine according toclaim 8, wherein the average thickness of the abradable layer is betweenabout 0.1 mm and about 0.9 mm.
 10. A centripetal turbine according toclaim 9, wherein the layer of abradable material has an averagethickness of about 0.4 mm.
 11. A centrifugal compressor comprising ahousing, a compressor wheel mounted within the housing and havingcompressor blades, the housing being provided with an annular layer ofan abradable material in a region adjacent said compressor blades,wherein the housing defines an inlet passageway which has a generallycylindrical portion arranged around a portion of the compressor wheel,an annular outlet passageway arranged around a portion of the compressorwheel, and a curved annular shoulder curving radially outwards from saidgenerally cylindrical portion of the inlet passageway to said annularoutlet passageway, the radially outer edge of each blade having a firstportion adjacent the generally cylindrical portion of the inletpassageway, and a second curved portion adjacent the curved annularshoulder, wherein the annular layer of abradable material covers atleast part of said curved shoulder but all, or substantially all, ofsaid cylindrical portion of the inlet passageway is left uncovered bysaid layer of abradable material.
 12. A centrifugal compressor accordingto claim 11, wherein the layer of abradable material covers only aregion of said annular shoulder in which the curvature of the shoulderhas an axial component which is greater than, or substantially equal to,its axial component.
 13. A centrifugal compressor according to claim 12,wherein the average thickness of the layer of abradable material isabout 0.1 mm less than the radial clearance between the compressor wheeland the housing in the region of the abradable layer.
 14. A centrifugalcompressor according to claim 13, wherein the average thickness of theabradable layer is between about 0.1 mm and 0.5 mm.
 15. A centrifugalcompressor according to claim 14, wherein the abradable materialcomprises a mixture of an aluminum alloy powder, silicon and polyester.16. A centrifugal compressor according to claim 15, wherein theabradable material comprises about 60% by weight of said aluminum alloypowder, about 12% by weight of silicon and about 28% by weight ofpolyester.
 17. A centrifugal compressor according to claim 16, whereinthe layer of abradable material is applied to the compressor housing bya plasma jet spray process.
 18. A turbine comprising a housing, aturbine wheel mounted within the housing and having turbine blades, thehousing being provided with an annular layer of an abradable material ina region adjacent said turbine blades, wherein said abradable materialcomprises about 93% by weight of nickel powder, about 5% aluminum powderby weight, and a binder.